Q-switched Erbium Doped Fiber Laser Generation Using the Kerr Effect of Multimode Interference

This work demonstrates a single mode fibre-step index multimode fibre-single mode fibre (SMS) structure as saturable absorber (SA) to generate Q-switched laser based on the Kerr-effect of multimode interference (MMI) in an Erbium-doped fibre laser (EDFL) cavity. A 156 mm length of step index multimode fibre (SIMF) was spliced directly into a ring cavity to modulate the cavity loss. By bending and applying proper pressure on the SIMF, steady Q-switched pulse was generated without strict length restriction of the SIMF. The Q-switched laser had a high repetition rate from 87 to 158 kHz, while the corresponding pulse width was low, ranging from 4.2 μs to 2.5 μs as the pump power increased from 115 to 342 mW. The signal-to-noise ratio (SNR) of the fundamental mode for the pulsed laser was about 67 dB at a central wavelength of 1530 nm. The experiment demonstrated that SMS structure can be used as stable SA to generate Q-switched laser for high power applications. It also indicated that even with large core diameter step index multimode fiber, SMS structure can be utilized to generate stable Q-switched pulsed laser with very simple cavity settings.


Introduction
Q-switched fibre lasers have gained tremendous interest due to their potential applications in remote sensing, medical surgery, photoacoustic imaging and material processing [1,2].Q-switched operation can be actively achieved using pulse modulators [3] or passively achieved using saturable absorbers (SAs) [4].Compared with active methods, passive methods are simple, compact, cost-effective, and satisfactory.According to reports, various materials including semiconductor saturated absorption mirrors (SESAM) [5], carbon nanotubes [1], graphene [6], transition metal dichalcogenide [7], and black phosphorous [8] were promising SAs.SESAM is characterized by a low damage threshold, narrow bandwidth, and high manufacturing cost, whereas other materials pose challenges in terms of fabrication and long-term stability.In recent years, single mode fiber-step index multimode fibersingle mode fiber (SMS) structure as SA has attracted attention due to its inherent high power damage threshold, wavelength independence, and low cost.The mechanism behind the SMS as SA is the nonlinear effect in the optical fiber-based laser cavity.Due to the effects of self-phase modulation (SPM) and cross-phase modulation (XPM), the beat length of self imaging under high intensity output is different from that under low intensity output.As a result, the modulation of cavity loss is utilized for the generation of Q-switched pulses [9].Many reports have shown successful mode-locked laser with SMS structure as SA [10,11], but only a few Q-switched laser with SMS structure as SA were reported.
In this work, we present a Q-switched erbium-doped fiber laser (EDFL) with a SMS structure as SA.The Q-switched laser pulse width decrease down to 2.5 µs when the output power of the laser pump reached our maximum of 343mW.The maximum repetition rate reached 158kHz at 343mW.We believe it is promising that the pulse width can be narrowed down to nanosecond level with higher pump power.

Experimental arrangement
The SMF-SIMF-SMF structure was fabricated by directly splicing a section of SIMF into the ring cavity.The splicer used was the Fujikura 90S+, and standard single mode fibre was used on both sides of the SIMF.Each splicing point had a loss of 0.01dB and 0.00dB respectively.The length of the SIMF was 156 mm, and its core and cladding diameters 105 µm and 125 µm, respectively.Figure 1(a) illustrates the schematic representation of the SMF-SIMF-SMF structure.The schematic diagram of experimental setup is shown in Figure 2. The continuous wave laser diode (LD) provided a maximum output power of 343 mW at 975 nm (II-VI, Model LC96Z400-74).The LD was spliced with a 980/1550 nm wavelength division multiplexer (WDM).A 2 m long Erbium-doped fibre (EDF, Fibercore I25) acting as the gain medium was connected to the cavity after the WDM.The output of gain medium was then connected to an isolator to ensure a unidirectional operation of the laser light.The SMS structure acting as the SA was fusion spliced into the cavity after the isolator.An 80:20 coupler was placed after the SMS as feedback to the cavity and output port.The total length of the cavity was about 9.6 m which included both the gain medium and the single-mode fibres.Due to Kerr effect, the transmission of the light depended on the refractive index, the length of the MMF and the center wavelength.However, the beat length of multimode fibers (MMF) primarily ranged in millimeters or even micrometers, making it challenging to achieve using the existing splicing technique.By applying stress on the fibre, the fibre shall be deformed and causing the refractive index change, so that the saturable absorption effect on MMI can be realized without strict restriction on the MMF length.According to this, constant weight was added to the fiber to decrease the accuracy requirements of the SIMF length.The SIMF was bent for the pulse generation.

Results and discussion
When the pump power was increased gradually, the free running of continuous wave (CW) laser was first observed.Q-switching operation started when pump power was set at 115 mW or higher.The characteristics of the Q-switched pulse laser are summarized in Figure 3.It operated at 1530 nm as shown in Figure 3(a).A typical pulse waveform at 115 mW is shown in Figure 3(b), which indicates the pulse period and duration of 11.44 μs and 4.2 μs, respectively.The repetition rate varied from 87.4 kHz to 157.7 kHz while the pulse width was reduced from 4.2 μs to 2.4 μs as the pump power was changed from 115 mW to 343 mW (Figure 3(c)).The pulse width decreased with reduced repetition rate and correspondingly increased output power.There was no trend of saturation in Figure 3(c), indicating the potential of producing higher repetition rate and lower pulse width with further power scale-up, which can be made by increasing the maximum power of the pump diode.
An SNR of 67 dB is observed in the RF spectrum, indicating the stability of the pulses as depicted in Figure 3(d).At least 13 harmonics were recorded within a frequency band of 2MHz.This further verifies the stability of the Q-switching operation.Mode-locked laser was also observed during the experiment with the modification of polarization in the laser cavity.During the experiments, the SMS structures were observed to be notably more sensitive to environment change than thin film SAs.
Because SMS structure can bear high power, we believe it is promising to get nanosecond Q-switched pulse with higher power.

Conclusion
We have successfully achieved a Q-switched fiber laser with the incorporation of structure in the EDFL cavity.The use of the SMF-SIMF-SMF as a saturable absorber in the cavity allowed for cost-effective and stable laser operation.Q-switching was accomplished by manipulating the SIMF through bending.The laser achieved a maximum pulse repetition rate of 157.7 kHz and a pulse width of 2.53 μ s.At a pump power of 343 mW, the laser output power reached 2.346 mW, resulting in a pulse energy of 14.9 nJ.Our results show that the SMF-SIMF-SMF structure can function as promising SA for short pulse generation.

Figure 1 .
Figure 1.(a) The SMS structure with 156 mm length of SIMF (b) The spectral transmission characteristic of the SIMF in the range from 1555 to 1580 nm.

Figure 2 .
Figure 2. Schematic diagram of experimental setup of the ring cavity

Figure 3 .
Figure 3. Q-switching performance (a) Optical spectrum at 343 mW pumping (b) Typical pulse train at 343 mW pumping (c) the plot of repetition rate and pulse width against pump power (d) RF spectrum.